ELECTRICALLY ISOLATED SiGe FIN FORMATION BY LOCAL OXIDATION

ABSTRACT

A silicon germanium alloy layer is formed on a semiconductor material layer by epitaxy. An oxygen impermeable layer is formed on the silicon germanium alloy layer. The oxygen impermeable layer and the silicon germanium alloy layer are patterned to form stacks of a silicon germanium alloy fin and an oxygen impermeable cap. A shallow trench isolation structure is formed by deposition, planarization, and recessing or an oxygen permeable dielectric material. An oxygen impermeable spacer is formed around each stack of a silicon germanium alloy fin and an oxygen impermeable cap. A thermal oxidation process is performed to convert a lower portion of each silicon germanium alloy fin into a silicon germanium oxide. During the thermal oxidation process, germanium atoms diffuse into unoxidized portions of the silicon germanium alloy fins to increase the germanium concentration therein.

BACKGROUND

The present disclosure relates to a semiconductor structure, andparticularly to a semiconductor structure including electricallyisolated SiGe fins, and a method for manufacturing the same.

A finFET is field effect transistor including a channel located in asemiconductor fin having a height that is greater than a width. FinFETsemploy vertical surfaces of semiconductor fins to effectively increase adevice area without increasing the physical layout area of the device.Fin based devices are compatible with fully depleted mode operation ifthe lateral width of the fin is thin enough. For these reasons, finbased devices can be employed in advanced semiconductor chips to providehigh performance devices.

Formation of a silicon germanium alloy fin including a highconcentration of germanium on a silicon substrate is difficult becauseof large lattice mismatch between silicon and germanium. As the atomicgermanium concentration increases in a silicon germanium alloy that isepitaxially formed on a silicon layer, defect density within the silicongermanium alloy also increases. Silicon germanium alloy fins patternedfrom the silicon germanium alloy also include high density of defects,which degrade performance of semiconductor devices formed on the silicongermanium alloy fins.

SUMMARY

A silicon germanium alloy layer is formed on a semiconductor materiallayer by epitaxy. An oxygen impermeable layer is formed on the silicongermanium alloy layer. The oxygen impermeable layer and the silicongermanium alloy layer are patterned to form stacks of a silicongermanium alloy fin and an oxygen impermeable cap. A shallow trenchisolation structure is formed by deposition, planarization, andrecessing or an oxygen permeable dielectric material. An oxygenimpermeable spacer is formed around each stack of a silicon germaniumalloy fin and an oxygen impermeable cap above the shallow trenchisolation structure. After deposition of a disposable oxygen permeablematerial between silicon germanium alloy fins, a thermal oxidationprocess is performed to convert a lower portion of each silicongermanium alloy fin into a silicon germanium oxide. During the thermaloxidation process, germanium atoms diffuse into unoxidized portions ofthe silicon germanium alloy fins to increase the germanium concentrationtherein, thereby providing silicon germanium alloy fins with a highergermanium concentration than the silicon germanium alloy layer as formedon the semiconductor material layer. Further, the thermal oxidationprocess electrically isolates each remaining portion of the silicongermanium alloy fins.

According to an aspect of the present disclosure, a semiconductorstructure is provided. The semiconductor structure includes asemiconductor oxide material portion that contains a semiconductor oxidelayer located on a semiconductor material layer and further contains aplurality of semiconductor oxide pedestals that protrudes above thesemiconductor oxide layer. A plurality of silicon germanium alloy finsis located on the plurality of semiconductor oxide pedestals. Each ofthe plurality of silicon germanium alloy fins is located directly on,and above, one of the plurality of semiconductor oxide pedestals. Ashallow trench isolation structure contacts a top surface of thesemiconductor oxide layer and sidewalls of the plurality ofsemiconductor oxide pedestals. Bottommost portions of the plurality ofsilicon germanium alloy fins are more distal from the semiconductoroxide layer than a planar top surface of the shallow trench isolationstructure is from the semiconductor oxide layer.

According to another aspect of the present disclosure, a method offorming a semiconductor structure is provided. A plurality of verticalstacks is formed on a semiconductor material layer. Each of theplurality of vertical stacks includes a silicon germanium alloy fin andan oxygen impermeable cap. A shallow trench isolation structurelaterally surrounding lower portions of the plurality of vertical stacksis formed directly on a top surface of the semiconductor material layer.The shallow trench isolation structure includes an oxygen permeablematerial. An oxygen impermeable spacer is formed directly on sidewallsof upper portions of the plurality of vertical stacks. An upper portionof the semiconductor material layer and lower portions of each of theplurality of silicon germanium alloy fins are oxidized employing anoxidation process. Sidewall surfaces of remaining portions of theplurality of silicon germanium alloy fins are physically exposed byremoving the plurality of oxygen impermeable spacers and the pluralityof oxygen impermeable caps.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a vertical cross sectional view of an exemplary semiconductorstructure after formation of a silicon germanium alloy layer and anoxygen impermeable layer on a semiconductor material layer according toan embodiment of the present disclosure.

FIG. 2 is a vertical cross sectional view of the exemplary semiconductorstructure after formation of silicon germanium alloy fins and oxygenimpermeable caps according to an embodiment of the present disclosure.

FIG. 2A is a top down view of the exemplary semiconductor structure ofFIG. 2. The vertical plane Z-Z′ represents the plane of the verticalcross sectional view of FIG. 2.

FIG. 3 is a vertical cross sectional view of the exemplary semiconductorstructure after an oxygen permeable dielectric material layer accordingto an embodiment of the present disclosure.

FIG. 4 is a vertical cross sectional view of the exemplary semiconductorstructure after recessing the oxygen permeable dielectric material layerto form a shallow trench isolation structure according to an embodimentof the present disclosure.

FIG. 5 is a vertical cross sectional view of the exemplary semiconductorstructure after formation of oxygen impermeable spacers according to anembodiment of the present disclosure.

FIG. 6 is a vertical cross sectional view of the exemplary semiconductorstructure after optional formation of a disposable oxygen permeablematerial layer according to an embodiment of the present disclosure.

FIG. 7 is a vertical cross sectional view of the exemplary semiconductorstructure after a thermal oxidation step that converts an upper portionof the semiconductor material layer and each bottom portion of thesilicon germanium alloy fins into a contiguous semiconductor oxideportion according to an embodiment of the present disclosure.

FIG. 8 is a vertical cross sectional view of the exemplary semiconductorstructure after removal of the optional disposable oxygen permeablematerial layer according to an embodiment of the present disclosure.

FIG. 9 is a vertical cross sectional view of the exemplary semiconductorstructure after removal of the oxygen impermeable spacers and the oxygenimpermeable caps according to an embodiment of the present disclosure.

FIG. 10 is a vertical cross sectional view of the exemplarysemiconductor structure after formation of a gate dielectric, a gateelectrode, and a gate spacer according to an embodiment of the presentdisclosure.

FIG. 10A is a top down view of the exemplary semiconductor structure ofFIG. 10. The vertical plane Z-Z′ represents the plane of the verticalcross sectional view of FIG. 10.

DETAILED DESCRIPTION

As stated above, the present disclosure relates to a semiconductorstructure including electrically isolated SiGe fins, and a method formanufacturing the same. Aspects of the present disclosure are nowdescribed in detail with accompanying figures. It is noted that like andcorresponding elements mentioned herein and illustrated in the drawingsare referred to by like reference numerals. As used herein, ordinalssuch as “first” and “second” are employed merely to distinguish similarelements, and different ordinals may be employed to designate a sameelement in the specification and/or claims.

Referring to FIG. 1, an exemplary semiconductor structure according toan embodiment of the present disclosure includes a semiconductormaterial layer 10. The semiconductor material layer 10 includes asemiconductor material, which can be an elemental semiconductormaterial, a compound semiconductor material, or an organic semiconductormaterial. In one embodiment, the semiconductor material layer 10includes a silicon-containing semiconductor material. Thesilicon-containing semiconductor material may be silicon, a silicongermanium alloy, a silicon carbon alloy, a silicon germanium carbonalloy, or a stack thereof. In one embodiment, the silicon-containingsemiconductor material can be single crystalline. In one embodiment, theentirety of the semiconductor material layer 10 can be singlecrystalline. The semiconductor material of the semiconductor materiallayer 10 may be intrinsic, p-doped, n-doped, or may be a combination ofdifferently doped regions. In one embodiment, the entirety of thesemiconductor material layer 10 can be a single crystalline siliconlayer.

A silicon germanium alloy layer 20L is formed on the top surface of thesemiconductor material layer 20L. The silicon germanium alloy layer 20Lcan be formed, for example, by epitaxy of a silicon germanium alloymaterial on the semiconductor material layer 10 employing an epitaxialdeposition process known in the art. Alternately, the silicon germaniumalloy layer 20L may be formed by implantation of germanium into asilicon-containing material within the semiconductor material layer 20L.In this case, the silicon-containing material within the semiconductormaterial layer 10 may be silicon or a silicon-germanium alloy.Optionally, a thermal anneal may be performed to heal crystallinedefects within the implanted region. The implanted region becomes thesilicon germanium alloy layer 20L.

The silicon germanium alloy layer 20L includes a greater atomicconcentration of germanium than the semiconductor material layer 20L. Inone embodiment, the atomic concentration of germanium within the silicongermanium alloy layer 20L can be in a range from 5% to 25%. In oneembodiment, the atomic concentration of germanium within the silicongermanium alloy layer 20L can be set at a highest level that does notintroduce crystalline defects within the silicon germanium alloy layer20L. Thus, the semiconductor material layer 10 does not includegermanium, or includes less germanium than the silicon germanium alloylayer 20L. The thickness of the silicon germanium alloy layer 20L can bein a range from 20 nm to 400 nm.

The silicon germanium alloy layer 20L can be single crystalline, and canbe epitaxially aligned to a single crystalline semiconductor materialwithin the semiconductor material layer 10. In one embodiment, thethickness of the silicon germanium alloy layer 20L can be less than thecritical thickness of the silicon germanium alloy material above whichthe silicon germanium alloy material would develop dislocations andother crystalline defects. In this case, the entirety of the silicongermanium alloy material within the silicon germanium alloy layer 20Lcan be pseudomorphic to the single crystalline structure of thesemiconductor material layer 10 without strain relaxation bydislocations, and the lattice parameters of the silicon germanium alloymaterial along the horizontal directions match the corresponding latticeparameters of the single crystalline material in the semiconductormaterial layer 10 along the horizontal directions.

Optionally, after forming the silicon germanium alloy layer 20L, asilicon cap (not shown) can be grown on top of the silicon germaniumalloy layer 20L. The optional silicon cap can help protecting thesilicon germanium alloy layer 20L during subsequent processing steps.For example, an oxygen impermeable material layer 30L can be formeddirectly on the silicon cap instead of the silicon germanium alloy layer20L. If formed, the silicon cap is subsequently incorporated into thesilicon germanium alloy layer 20L because Ge in the underlying silicongermanium alloy layer 20L will diffuse upwards into the silicon cap.

An oxygen impermeable material layer 30L is formed over the stack of thesemiconductor material layer 10 and the silicon germanium alloy layer20L. As used herein, an “oxygen impermeable” element refers to anelement composed of a material having an oxygen diffusion rate that doesnot exceed 1/10 of the oxygen diffusion rate of a silicon nitridematerial formed by low pressure chemical vapor deposition (LPCVD) withina temperature range between 600 degrees Celsius and 1,000 degreesCelsius. The oxygen impermeable material layer 30L can include a nitrideof a semiconductor material or a nitride of a metallic material. Forexample, the oxygen impermeable material layer 30L can include siliconnitride or tantalum nitride. The oxygen impermeable material layer 30Lcan be deposited, for example, by chemical vapor deposition (CVD),physical vapor deposition (PVD), or atomic layer deposition (ALD). Thethickness of the oxygen impermeable material layer 30L can be in a rangefrom 3 nm to 60 nm, although lesser and greater thicknesses can also beemployed.

Referring to FIGS. 2 and 2A, the oxygen impermeable material layer 30Land the silicon germanium alloy layer 20L can be patterned into aplurality of vertical stacks, each including a silicon germanium alloyfin 20 and an oxygen impermeable cap 30 and the optional silicon cap.The patterning of the oxygen impermeable material layer 30L and thesilicon germanium alloy layer 20L can be performed, for example, byapplying and lithographically patterning a photoresist layer (not shown)over the top surface of the oxygen impermeable material layer 30L, andtransferring the pattern in the photoresist layer through the oxygenimpermeable material layer 30L and the silicon germanium alloy layer 20Lby an anisotropic etch that employs the patterned photoresist layer asan etch mask.

The anisotropic etch may, or may not, be selective to the semiconductormaterial of the semiconductor material layer 10. In one embodiment, theanisotropic etch can be selective to the semiconductor material of thesemiconductor material layer 10. In another embodiment, top surfaces ofthe semiconductor material layer 10 may be vertically recessed betweeneach adjacent pair of vertical stacks (20, 30) of the silicon germaniumalloy fins 20 and oxygen impermeable caps 30. In yet another embodiment,a horizontal bottom portion of the silicon germanium alloy layer 20L mayremain between each adjacent pair of vertical stacks (20, 30) of thesilicon germanium alloy fins 20 and oxygen impermeable caps 30. Acontiguous trench 11 laterally surrounds the plurality of verticalstacks (20, 30) of a silicon germanium alloy fin 20 and an oxygenimpermeable cap 30. The photoresist layer is subsequently removed, forexample, by ashing. Alternatively, sidewall imaging transfer techniquecan be used for forming the silicon germanium alloy fins 20.

As used herein, a “semiconductor fin” refers to a contiguous structureincluding a semiconductor material and including at least one pair ofsubstantially vertical sidewalls that are parallel to each other. Asused herein, a surface is “substantially vertical” if there exists avertical plane from which the surface does not deviate by more thanthree times the root mean square roughness of the surface. As usedherein, a “silicon germanium alloy fin” refers to a semiconductor fincomposed of a silicon germanium alloy material.

In one embodiment, within each vertical stack of a silicon germaniumalloy fin 20 and an oxygen impermeable cap 30, sidewalls of the silicongermanium alloy fin 20 can be vertically coincident with sidewalls ofthe oxygen impermeable cap 30. As used herein, two surfaces are“vertically coincident” if there exists a vertical plane including thetwo surfaces.

In one embodiment, the oxygen impermeable caps 30 can have rectangularhorizontal cross sectional areas. The horizontal direction along whichlonger sides of each rectangle extend is herein referred to as alengthwise direction of the corresponding oxygen impermeable cap 30. Thehorizontal direction that is perpendicular to the lengthwise directionof an oxygen impermeable cap 30 is herein referred to as a widthwisedirection of the corresponding oxygen impermeable cap 30. In oneembodiment, the oxygen impermeable caps 30 can be arranged as a lineararray in which oxygen impermeable caps 30 having a same rectangularcross sectional area are periodically placed along a common widthwisedirection of the oxygen impermeable caps 30. In one embodiment, theshapes of each overlying oxygen impermeable cap 30 can be replicated inan underlying silicon germanium alloy fin 20.

Referring to FIG. 3, an oxygen permeable dielectric material isdeposited in the contiguous trench 11 to fill the contiguous trench 11.As used herein, an “oxygen permeable dielectric material” refers to adielectric material composed of a dielectric material having an oxygendiffusion rate greater than 0.8 times the oxygen diffusion rate ofthermal silicon oxide within a temperature range between 600 degreesCelsius and 1,000 degrees Celsius. The oxygen permeable dielectricmaterial can be, for example, undoped silicate glass, fluorosilicateglass, phosphosilicate glass, or a spin-on glass (SOG) material. Theoxygen permeable dielectric material can be deposited by chemical vapordeposition, atomic layer deposition, or spin coating. The oxygenpermeable dielectric material can be deposited to fill the entirety ofthe contiguous trench 11, and any portion of the oxygen permeabledielectric material above a horizontal plane including the top surfacesof the oxygen permeable caps 30 can be removed by planarization. Forexample, chemical mechanical planarization (CMP) can be employed toremove excess portions of the deposited oxygen permeable dielectricmaterial from above the horizontal plane including the top surfaces ofthe oxygen permeable caps 30 can be removed by planarization. Remainingportions of the oxygen permeable dielectric material constitutes ashallow trench isolation (STI) dielectric material layer 40L.

Referring to FIG. 4, the oxygen permeable dielectric material of the STIdielectric material layer 40L is vertically recessed by an etch that isselective to the material of the oxygen impermeable caps 30. In oneembodiment, the etch can be an isotropic etch that is selective to thematerial of the oxygen impermeable caps 30 and the material of thesilicon germanium alloy fins 20. For example, the etch can be a wet etchemploying hydrofluoric acid. In another embodiment, the etch can be ananisotropic etch such as a reactive ion etch.

Remaining portions of the STI dielectric material layer 40L after theetch constitutes an STI structure 40, which is a contiguous structureincluding the oxygen permeable dielectric material and laterallysurround lower portions of each silicon germanium alloy fin 20. Thethickness of the STI structure 40 can be in a range from 5% of theheight of the silicon germanium alloy fins to 60% of the height of thesilicon germanium alloy fins. Further, the thickness of the STIstructure 40 can be in a range from 4 nm to 100 nm, although lesser andgreater thicknesses can also be employed. A contiguous trench 41overlying the STI structure 40 laterally surrounds upper portions of thesilicon germanium alloy fins 20 and all of the oxygen impermeable caps30. In one embodiment, the STI structure 40 can laterally surround lowerportions of the plurality of vertical stacks (20, 30) located directlyon the top surface of the semiconductor material layer 10.

Referring to FIG. 5, oxygen impermeable spacers 32 are formed directlyon sidewalls of upper portions of the plurality of vertical stacks (20,30) of a silicon germanium alloy fin 20 and an oxygen impermeable cap30. The oxygen impermeable spacers 32 can be formed, for example, bydepositing a conformal oxygen impermeable material layer employingchemical vapor deposition (CVD) or atomic layer deposition (ALD), and byanisotropically etching horizontal portions of the conformal oxygenimpermeable material layer by an anisotropic etch. Horizontal portionsof the conformal oxygen impermeable material layer are removed by theanisotropic etch, and remaining vertical portions of the conformaloxygen impermeable material layer constitute the oxygen impermeablespacers 32. The anisotropic etch may be selective, or non-selective tothe oxygen permeable dielectric material of the STI structure 40. Theselectivity of the anisotropic etch is selected such that the STstructure 40 is not punched through during the anisotropic etch. In oneembodiment, the anisotropic etch may be selective to the oxygenpermeable material of the STI structure 40.

In one embodiment, the oxygen impermeable spacer 32 includes adielectric nitride of a semiconductor material such as silicon nitrideor a metallic nitride such as titanium nitride. The thickness of theoxygen impermeable spacers 32 is less than one half of the minimumspacing between a neighboring pair of the plurality of vertical stacks(20, 30) of a silicon germanium alloy fin 20 and an oxygen impermeablecap 30.

Referring to FIG. 6, a disposable oxygen permeable material layer 50 mayoptionally be formed within the contiguous trench 41 by depositing anoxygen permeable material therein. The disposable oxygen permeablematerial layer 50 is an oxygen permeable material layer that is removedin a subsequent processing step. The oxygen permeable material layer 50is formed above the shallow trench isolation structure 40 and betweenthe plurality of vertical stacks (20, 30). The oxygen permeabledielectric material is deposited to a height above topmost surfaces ofthe plurality of vertical stacks (20, 30).

The disposable oxygen permeable material layer 50 includes an oxygenpermeable dielectric material such as undoped silicon oxide,borosilicate glass (BSG), fluorosilicate glass (FSG),borophosphosilicate glass (BPSG), or a spin-on glass material. Thedisposable oxygen permeable material layer 50 can be formed by chemicalvapor deposition (CVD) or spin coating.

In one embodiment, an entirety of the top surface of the disposableoxygen permeable material layer 50 can be located above the horizontalplane including the top surfaces of the oxygen impermeable caps 30. Inanother embodiment, portions of the disposable oxygen permeable materiallayer 50 above the horizontal plane including the top surfaces of theoxygen impermeable caps 30 can be removed by planarization, for example,by chemical mechanical planarization or a recess etch. In this case, thetop surface of the disposable oxygen permeable material layer 50 may becoplanar with the top surfaces of the oxygen impermeable caps 30.

In one embodiment, the oxygen permeable material within the disposableoxygen permeable material layer 50 can be different from the oxygenpermeable material within the STI structure 40. In one embodiment, theoxygen permeable material within the disposable oxygen permeablematerial layer 50 can have a greater etch rate than the oxygen permeablematerial within the STI structure 40 in an etch chemistry such as a wetetch chemistry employing hydrofluoric acid. In one embodiment, the STIstructure 40 can include undoped silicate glass, and the disposableoxygen permeable material layer 50 can include borosilicate glass.

Referring to FIG. 7, a thermal oxidation process is performed to oxidizeupper portion of the semiconductor material layer 10 and bottom portionsof each silicon germanium alloy fin 20 that are proximal to the shallowtrench isolation structure 40. During the thermal oxidation process,oxidizing species diffuse through the disposable oxygen permeablematerial layer 50 and through the shallow trench isolation structure 40,and enter the upper portion of the semiconductor material layer 10 andbottom portions of each silicon germanium alloy fin 20 (See FIG. 6). Theoxidizing species may be oxygen molecules, water molecules, oxygenplasma, oxygen ions, or any other species that can form a semiconductoroxide upon reaction with a semiconductor material. The oxidizing speciesreact with the semiconductor material within the upper portion of thesemiconductor material layer 10 and the bottom portions of the silicongermanium alloy fins 20 to form a contiguous semiconductor oxide portion60. The oxygen impermeable caps 30 and the oxygen impermeable spacers 32prevent diffusion of oxidizing species into the sidewalls or the topsurfaces of the silicon germanium alloy fins 20.

The processing conditions and the duration of the thermal oxidationprocess can be selected such that entirety of the contiguoussemiconductor oxide portion 60 is formed as a single contiguousstructure, and an upper portion of each silicon germanium alloy fin 20is not converted into a semiconductor oxide material. During thermaloxidation, germanium atoms are pushed into a remaining unoxidizedsilicon germanium alloy material portion from an interface between theunoxidized silicon germanium alloy material portion and an oxidizedsilicon germanium oxide material portion. In other words, the oxidationprocess drives germanium atoms from oxidized portions of the pluralityof silicon germanium alloy fins 22 into unoxidized remaining portions ofthe plurality of silicon germanium alloy fins 22. Thus, each remainingupper portion of the silicon germanium alloy fins 20 includes a greateratomic concentration after the thermal oxidation process than prior tothe thermal oxidation process, and is herein referred to as a germaniumenriched silicon germanium alloy fin 22, i.e., a silicon germanium alloyfin having a greater atomic concentration of germanium compared to thesilicon germanium alloy fins 20 prior to the thermal oxidation process.

During the thermal oxidation process, the presence of the disposableoxygen permeable material layer 50 provides mechanical support to eachvertical stack (20, 30) of a silicon germanium alloy fin 20 and anoxygen impermeable cap 30 or to each vertical stack (22, 30) of agermanium-enriched silicon germanium alloy fin 22 and an oxygenimpermeable cap 30. Thus, presence of the disposable oxygen permeablematerial layer 50 during the thermal oxidation process prevents lateralshifting or tilting of the vertical stack (22, 30) of agermanium-enriched silicon germanium alloy fin 22 and an oxygenimpermeable cap 30. In an embodiment in which a disposable oxygenpermeable material layer 50 is not present, the thermal oxidationprocess may proceed by diffusing oxidizing species through the shallowtrench isolation structure 40.

Referring to FIG. 8, the optional disposable oxygen permeable materiallayer 50 is removed, for example, by an isotropic etch, an anisotropicetch or combination of both. In one embodiment, a wet etch employinghydrofluoric acid can be employed to remove the disposable oxygenpermeable dielectric layer 50. The removal of the disposable oxygenpermeable dielectric layer 50 may be selective, non-selective, or partlyselective to the dielectric material of the shallow trench isolationstructure 40. While the present disclosure is described employing anembodiment in which removal of the disposable oxygen permeabledielectric layer 50 is selective to the dielectric material of theshallow trench isolation structure 40, embodiments in which the removalof the disposable oxygen permeable dielectric layer 50 is non-selective,or partly selective, to the dielectric material of the shallow trenchisolation structure 40 can also be employed. In this case, a remainingportion of the disposable oxygen permeable material layer 50 may bepresent on the top surface of the shallow trench isolation structure 40due to an underetch, or the top surface of the shallow trench isolationstructure 40 may be recessed due to an overetch.

Referring to FIG. 9, the oxygen impermeable spacers 32 and the oxygenimpermeable caps 30 can be removed selective to the dielectric materialsof the shallow trench isolation structure 40 and the contiguoussemiconductor oxide portion 60. The removal of the oxygen impermeablespacers 32 and the oxygen impermeable caps 30 can be performed by anisotropic etch. For example, if the oxygen impermeable spacers 32 andthe oxygen impermeable caps 30 include silicon nitride and if theshallow trench isolation structure 40 includes silicon oxide, theremoval of the oxygen impermeable spacers 32 and the oxygen impermeablecaps 30 can be performed by a wet etch process employing hot phosphoricacid. Sidewall surfaces and top surfaces of the germanium-enrichedsilicon germanium alloy fin 22 (which are the remaining portions of theplurality of silicon germanium alloy fins 20) are physically exposed byremoving the plurality of oxygen impermeable spacers 32 and theplurality of oxygen impermeable caps 30.

In the exemplary semiconductor structure, the semiconductor oxidematerial portion 60 containing a semiconductor oxide layer 60L locatedon the semiconductor material layer 10 and a plurality of semiconductoroxide pedestals 60P that protrudes above the semiconductor oxide layer60L. As used herein, a “pedestal” refers to a portion of a structurethat protrudes from a planar surface of an underlying portion of thestructure. The germanium-enriched silicon germanium alloy fins 22 are aplurality of silicon germanium alloy fins located on the plurality ofsemiconductor oxide pedestals 60P. Each of the plurality of silicongermanium alloy fins is located directly on, and above, one of theplurality of semiconductor oxide pedestals 22.

The shallow trench isolation structure 40 contacts a top surface of thesemiconductor oxide layer 60L and sidewalls of the plurality ofsemiconductor oxide pedestals 60P. In one embodiment, bottommostportions of the plurality of germanium-enriched silicon germanium alloyfins can be more distal from the semiconductor oxide layer 60L than aplanar top surface of the shallow trench isolation structure 40 is fromthe semiconductor oxide layer 60L. Each of the plurality ofgermanium-enriched silicon germanium alloy fins 22 includes a pair ofconcave bottom surfaces that contact surfaces of a semiconductor oxidepedestal 60P. The pair of concave surfaces is adjoined at an edge thatis parallel to a pair of vertical sidewalls of one of the plurality ofgermanium-enriched silicon germanium alloy fins 22.

Because oxidation of the upper portion of the semiconductor materiallayer 10 proceeds from the interface with the shallow trench isolationstructure 40 during the thermal oxidation process, portions of theinterface between the semiconductor oxide layer 60L and thesemiconductor material layer 10 protrude downward in regions that do notunderlie any of the plurality of germanium-enriched silicon germaniumalloy fins 22 with respect to other portions of the interface in regionsthat underlie the plurality of germanium-enriched silicon germaniumalloy fins 20.

Because the semiconductor material layer 10 has a lesser atomicconcentration or germanium than the silicon germanium alloy fins 20prior to the thermal oxidation process, the plurality of semiconductoroxide pedestals 60P has a greater concentration of germanium atoms thanthe semiconductor oxide layer 60L. Further, because the semiconductormaterial layer 10 has a lesser atomic concentration or germanium thanthe silicon germanium alloy fins 20 prior to the thermal oxidationprocess, the ratio of germanium atoms to silicon atoms in the pluralityof semiconductor oxide pedestals 60F is greater than the ratio ofgermanium atoms to silicon atoms in the semiconductor oxide layer 10.

In addition, because germanium atoms are pushed into unoxidizedremaining portions of the silicon germanium alloy fins 20 during thethermal oxidation process, the ratio of germanium atoms to silicon atomsin the plurality of germanium-enriched silicon germanium alloy fins 22is greater than the ratio of germanium atoms to silicon atoms in theplurality of semiconductor oxide pedestals 60P. This feature can beadvantageously employed to provide single crystalline germanium-enrichedsilicon germanium alloy fins 22 with a higher atomic concentration ofgermanium than is possible by epitaxial deposition of a singlecrystalline silicon germanium only. Specifically, increase in the atomicconcentration of germanium in the germanium-enriched silicon germaniumalloy fins 22 does not generate crystalline defects within thegermanium-enriched silicon germanium alloy fins 22 because thegermanium-enriched silicon germanium alloy fins 22 are not epitaxiallyaligned to the semiconductor material layer 10 due to formation of thesemiconductor oxide material portion 60. By increasing the atomicconcentration of germanium while the semiconductor oxide materialportion 60 removes the epitaxial alignment between the semiconductormaterial layer 10 and the germanium-enriched silicon germanium alloyfins 22, formation of dislocations and other crystalline defects in thegermanium-enriched silicon germanium alloy fins 22 is avoided.

In one embodiment, the silicon germanium alloy layer 20L and thesemiconductor material layer 10 can have an identical crystallographicstructure, and can be epitaxially aligned to each other. Thus, theplurality of germanium-enriched silicon germanium alloy fins 22 and thesemiconductor material layer 10 can have an identical crystallographicstructure, and can have an identical spatial orientation for eachcrystallographic axis of the identical crystallographic structure.

Referring to FIGS. 10 and 10A, a gate structure (70, 72) can be formedacross the plurality of germanium-enriched silicon germanium alloy fins22. The gate structure (70, 72) can straddle one or more of theplurality of germanium-enriched silicon germanium alloy fins 22.Optionally, a gate spacer 76 can be formed around the gate structure(70, 72). Various portions of the plurality of germanium-enrichedsilicon germanium alloy fins 22 can be doped to form source regions anddrain regions to provide a fin field effect transistor.

While the disclosure has been described in terms of specificembodiments, it is evident in view of the foregoing description thatnumerous alternatives, modifications and variations will be apparent tothose skilled in the art. Each of an embodiments described herein can beimplemented individually or in combination with any other embodimentunless expressly stated otherwise or clearly incompatible. Accordingly,the disclosure is intended to encompass all such alternatives,modifications and variations which fall within the scope and spirit ofthe disclosure and the following claims.

1.-10. (canceled)
 11. A method of forming a semiconductor structurecomprising: forming a plurality of vertical stacks on a semiconductormaterial layer, each of said plurality of vertical stacks including asilicon germanium alloy fin and an oxygen impermeable cap; forming ashallow trench isolation structure laterally surrounding lower portionsof said plurality of vertical stacks, said shallow trench isolationstructure comprising an oxygen permeable material; forming an oxygenimpermeable spacer directly on sidewalls of upper portions of saidplurality of vertical stacks; oxidizing an upper portion of saidsemiconductor material layer and lower portions of each of saidplurality of silicon germanium alloy fins employing an oxidationprocess; and physically exposing sidewall surfaces of remaining portionsof said plurality of silicon germanium alloy fins by removing saidplurality of oxygen impermeable spacers and said plurality of oxygenimpermeable caps.
 12. The method of claim 11, further comprising formingan oxygen permeable material layer above said shallow trench isolationstructure and between said plurality of vertical stacks prior to saidoxidation process.
 13. The method of claim 12, further comprisingremoving said oxygen permeable material layer after said oxidationprocess.
 14. The method of claim 12, wherein said oxygen permeablematerial layer is formed by depositing an oxygen permeable dielectricmaterial on said shallow trench isolation structure to a height abovetopmost surfaces of said plurality of vertical stacks.
 15. The method ofclaim 11, wherein said semiconductor material layer does not includegermanium, or includes less germanium than said plurality of silicongermanium alloy fins.
 16. The method of claim 11, wherein said oxidationprocess drives germanium atoms from oxidized portions of said pluralityof silicon germanium alloy fins into unoxidized remaining portions ofsaid plurality of silicon germanium alloy fins.
 17. The method of claim11, wherein said plurality of vertical stacks is formed by: providing amaterial stack including a silicon germanium alloy layer and an oxygenimpermeable material layer on said semiconductor material layer; andpatterning said material stack, wherein remaining portions of saidmaterial stack comprise said plurality of vertical stacks.
 18. Themethod of claim 17, wherein said silicon germanium alloy layer is formedby epitaxial deposition of a silicon germanium alloy material on asingle crystalline semiconductor material in said semiconductor materiallayer.
 19. The method of claim 18, wherein said semiconductor materiallayer is a single crystalline silicon layer.
 20. The method of claim 11,further comprising forming a gate structure straddling said remainingportions of said plurality of silicon germanium alloy fins, said gatestructure comprising a gate dielectric and a gate electrode.
 21. Themethod of claim 11, wherein said forming said shallow trench isolationstructure comprises depositing a dielectric material layer between eachvertical stack and recessing said dielectric material layer to providesaid shallow trench isolation structure, said shallow trench isolationstructure having a height that is less than a height of said verticalstack.
 22. The method of claim 11, wherein said oxygen impermeablespacer is formed on a topmost surface of said shallow trench isolationstructure.
 23. The method of claim 11, wherein said upper portion ofsaid semiconductor material layer is converted into a contiguoussemiconductor oxide portion having a semiconductor oxide pedestalportion located beneath each silicon germanium alloy fin.
 24. The methodof claim 23, wherein each of said plurality of silicon germanium alloyfins comprises a pair of concave bottom surfaces that contact surfacesof said semiconductor oxide pedestal.
 25. The method of claim 24,wherein said pair of concave surfaces is adjoined at an edge that isparallel to a pair of vertical sidewalls of one of said plurality ofsilicon germanium alloy fins.
 26. The method of claim 23, wherein aportion of an interface between said contiguous semiconductor oxideportion and said semiconductor material layer protrudes downward inregions that do not underlie any of said plurality of silicon germaniumalloy fins with respect to another portion of said interface in regionsthat underlie said plurality of silicon germanium alloy fins.
 27. Themethod of claim 23, wherein each semiconductor oxide pedestal has agreater concentration of germanium atoms that a portion of saidcontiguous semiconductor oxide portion that is located beneath said eachsemiconductor oxide pedestal.
 28. The method of claim 23, wherein aratio of germanium atoms to silicon atoms in said plurality of silicongermanium alloy fins is greater than a ratio of germanium atoms tosilicon atoms in each semiconductor oxide pedestal.